Recently, next generation mobile communication systems have actively been researched, and a single frequency reuse cellular system in which the same frequency band is shared by multiple cells has been proposed as a method of enhancing the system frequency utilization efficiency.
OFDMA (Orthogonal Frequency Division Multiple Access) is most popular for downlink communication (from a base station device to a mobile station). In an OFDMA communication system, modulation, such as 64 QAM (64 Quadrature Amplitude Modulation) or BPSK (Binary Phase Shift Keying), is performed on information data to form OFDM signals to be used for communication. Then, a resource block which is an access unit defined by time and frequency axes is divided and assigned to multiple mobile terminal devices. Since OFDM signals are used, PAPR (Peak to Average Power Ratio) occasionally becomes very high. The high peak power does not cause a significant problem in downlink communication since a transmission power amplifying function is sufficiently performed in downlink. However, the high peak power causes a crucial problem in uplink communication (from a mobile station to a base station device) since the transmission power amplifying function is not sufficiently performed in uplink.
For this reason, single carrier communication systems in which PAPR is relatively small have been proposed for uplink communication, one of which is DFT-s-OFDM (Discrete Fourier Transform-spread-OFDM) (see Non-Patent Document 1). FIG. 24 is a transmitter block diagram. An encoder 111 performs, on input transmission data, error correction coding and then modulation such as BPSK to generate a time domain signal. Then, an S/P (Serial/Parallel) converter 101 converts the time domain signal into parallel signals. Then, a AFT (Discrete Fourier Transform) unit 102 performs a Fourier transform to convert the time domain signals into frequency signals, which are input to an IDFT (Inverse Discrete Fourier Transform) unit 105 through a subcarrier allocator 104 based on a rule which will be explained later. A 0 is assigned to each IDFT point having no input, and then IDFT is performed to generate a time waveform. Then, a GI (Guard Interval) inserter 106 inserts a guard interval into the time waveform. Then, a P/S (Parallel/Serial) converter 107 converts the waveform into a serial signal. Then, a D/A (Digital/Analog) converter 108 converts the serial signal into an analog signal. Then, an RF (radio frequency) unit 109 upconverts the analog signal into a radio frequency signal to be transmitted through an antenna (not shown). In a system in which multiple user data are multiplexed, the IDFT point number is set to be greater than the DFT point number, and subcarriers to which 0s are assigned are used by another mobile terminal device.
The data generated in this manner have small PAPR similarly to single carrier modulation. Further, frequency domain control can easily be performed since a frequency waveform is preliminarily generated by DFT.
Two frequency allocation methods have been proposed. One is L (Localized) allocation, and the other is D (Distributed) allocation. The L allocation is illustrated in FIG. 25(a) in which frequency data subjected to DFT is successively allocated to inputs of IDFT without changing the allocation of the frequency data. The D allocation is shown in FIG. 25(b) in which the same data is separately allocated at a given interval to the inputs of IDFT.
The L allocation achieves a diversity effect by each user selecting an adequate frequency band, i.e., a user diversity effect. The D allocation achieves the frequency diversity effect since a broader frequency band is used. However, subcarriers optimal for communication are not selected in both methods. Therefore, sufficient performance cannot be achieved especially in a channel condition in which frequency selectivity is strong or in a condition in which there are many interference signals from other cells.
On the other hand, single CI (Carrier Interferometry) has been proposed as a similar uplink communication system (see Non-Patent Document 2). In this method, transmission signals can be generated by the same signal generating method as DFT-s-OFDM. This reference document suggests an allocation rule more flexible than the aforementioned allocation rule.
In this method, frequency signals subjected to DFT are segmented into a few subcarriers, and subcarriers less affected by other cells are selected when allocated to the inputs of the IDFT unit (hereinafter, LS allocation). Thereby, subcarriers can be selected with higher communication precision compared to the aforementioned L allocation.
Additionally, an increase in PAPR can be reduced by increasing the number of frequency signals included in a cluster. Further, optimal subcarriers can be selected when the number of frequency signals in a cluster is assumed to be 1 (it is defined as R allocation since subcarriers are randomly allocated to the inputs of IDFT).
FIG. 26 illustrates an example of a PAPR distribution of outputs of the IDFT unit 105 in those methods. The horizontal and vertical axes denote PAPR (dB) per symbol and cumulative distribution (%), respectively, where the DFT point number is 16, the IDFT point number is 64, and time domain data is modulated based on BPSK. The PAPR denotes values compared to outputs of the IDFT unit 105. In FIG. 26, L, D, and R denote the L allocation, the D allocation, and the R allocation, respectively. S denotes an example of the LS allocation. The number of frequency signals in one cluster is assumed to be 4 in the LS allocation. FIG. 27 illustrates subcarriers to be used for the respective allocations.
As can be understood from the illustration, the L and D allocations have no difference in the PAPR characteristics. The R allocation has the greatest PAPR, and the LS allocation has the middle PAPR between that of the L (D) allocation and that of the R allocation.
FIG. 28 illustrates a PAPR distribution when the number of frequency signals included in a cluster, i.e., the number of subcarriers, is changed in the LS allocation. As shown in the subcarrier allocations in FIG. 29, the number of subcarriers for LS1 is 1 (identical to that for the R allocation). The number of subcarriers for LS2, LS4 (identical to that for the LS allocation shown in FIG. 26), and LS8 are 2, 4, and 8, respectively. The number of subcarriers for LS16 is 16, which is identical to that for the L allocation. As can be understood from FIG. 28, the greater the number of frequency signals included in a cluster is, the smaller the PAPR is.
In the present description, communication methods of generating single carrier signals by a multi-carrier signal generating method, such as DFT-s-OFDM or CI, and of controlling the generated spectra for communication are collectively called SC^2 (Spectrum Controlled Carrier Transmission).    Non-Patent Document 1: 3GPP R1-050702 “DFT-Spread OFDM with Pulse Shaping Filter in Frequency Domain in Evolved UTRA Uplink” NTT DoCoMo    Non-Patent Document 2: The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Communications (PIMRC '06) “MICROSCOPIC SPECTRUM CONTROL TECHNIQUE USING CARRIER INTERFEROMETRY FOR ONE-CELL REUSE SINGLE CARRIER TDMA SYSTEM” Osaka University